Recently, new dielectric materials have attracted immense attention since they can be used to fabricate multi-layer microwave chips and devices. Dielectric materials should have a sintering temperature lower than the melting point (Tm=940° C.) of internal electrodes formed of metals such as Ag for their application in, for example, chips having internal Ag contacts, together with minimum dielectric losses.
Currently, low temperature co-fired ceramics (LTCC) have been prepared by firing a mixture of conventional dielectric materials having a sintering temperature Ts>1100° C., and considerable amounts of glass frits or the components thereof. However, reducing the sintering temperature of such materials to about 900° C. is usually accompanied by rather high dielectric losses (low Q value, Q=1/tan δ) at microwave frequencies.
For example, U.S. Pat. No. 5,759,935 discloses that dense microwave dielectric ceramics can be prepared by mixing 10% of glass frit, which is obtained from a glass composition comprising PbO, SiO2, B2O3 and ZnO, with a Zn- or Ta-doped BaO-xTiO2 composition (3≦x≦5.5), and then sintering the mixture at a temperature of about 900° C. for 2 hours in air. However, the Q·f0 values of such materials thus obtained do not exceed 2,500–3,500 GHz.
Another LTCC development is based on the introduction of small amounts (e.g., 0.1–5%) of dopant into the conventional microwave dielectric compound having a sintering temperature of about 1100 to 1300° C. This method allows the sintering temperature to be lowered by 200–300° C. by selecting suitable dopants. However, it has a problem in that a significant increase of dielectric loss will also result, even when using small amounts of dopant.
Alternative LTCC materials such as CaTeO3 and Bi6Te2O15 have been developed (see M. Valant et al., J. Eur. Ceram. Soc., 24 (2004), pp. 1715–1719; M. Udovic et al., J. Am. Ceram. Soc., 87 (2004), pp. 891–897). Such dielectric ceramics can be sintered at 900° C. or less without sintering aids or glass frits. However, they are disfavored since they contain toxic and volatile tellurium oxide.
Further, one of the conventional ways of reducing the sintering temperature of dielectric materials is to employ finely dispersed precursor powders obtained by the wet chemical methods (see Li-Wen Chu et al., Mater. Chem. Phys., 79, 2003, pp. 276 –281; M. H. Weng et al., J. Eur. Ceram. Soc., 22, 2002, pp. 1693–1698). Such methods can be successfully applied for producing microwave dielectrics having high Q·f0 values (100,000–500,000 GHz). These materials, however, can be obtained only at a high sintering temperature (1,400–1,700° C.) due to the refractory components therein. The sintering temperature can be reduced by about 100–300° C. by using chemically derived precursors. Such chemically derived precursors have high chemical homogeneity and lead to significant grain-growth during the sintering process, thereby resulting in decrease of dielectric losses caused by insufficient crystallographic ordering and defects at the grain boundaries. However, the application of chemically derived precursors to LTCC (Ts<900° C.) dielectrics is hampered by enhanced energy dissipation at the grain boundaries. In most cases, the conventional method using a chemically derived precursor would produce dielectric ceramics having moderate or low Q·f0 values (see H. Wang et al., Solid State Comm., 132(7), 2004, pp. 481–486).
In addition, Zn3Nb2O8 ceramics, which have Q·f0 values up to 80,000 GHz at εr=22–23, have been developed. In this case, the sintering temperature of the ceramics can be reduced to 850° C. by using a sintering aid which melts at a low temperature (see Yen-Chi Lee et al., Mater. Chem. Phys., 79, 2003, pp. 124–128).
Further, BiNbO4 is another candidate for a low-temperature sintered dielectric material. For example, Japanese Patent Nos. 07172916 and 05074225 disclose that the sintering temperature can be reduced to 875° C. through using suitable sintering aids.
However, those methods prepare the Nb-based ceramics through using coarsely grained precursor-powders obtained by using a solid-state synthesis technique, which has a limitation in applying for the preparation of low-temperature sintered dielectric ceramics. This is because the solid-state synthesis technique forms a precursor-powder in rather large grain sizes (e.g., usually several microns) so as to prevent the mobility thereof at a low sintering temperature, thus causing poor sinterability.